structure-sensitive investigations on glass fibers from the system SiO₂-Al₂O₃-GaO

Abstract

Glass fibers of the system SiO₂-Al₂O₃-CaO were prepared with respect to defined drawing conditions and investigated with structure-sensitive methods (birefringence, radial and axial alterations by thermal treatment at Tg). These glass fibers, which lie in a compositional sense roughly between the disilicate and silica glasses concerning the ratio network modifiers to network formers, exhibit similar optical anisotropies. On the other hand, they show remarkably lower axial and radial alterations which are exclusively contracdons on annealing for all applied drawing conditions in contrast to the silica or the alkali disilicate glass fibers. Obviously, the filling of free volume hollows with and the reinforcement of them by the Ca²⁺ ions and the smaller polarizability of the latter as compared to alkali ions are responsible for the relatively low structural anisotropy. An increase of the optical anisotropy with increasing CaO concentration is observed at comparable viscosities. This is a consequence of the increasing incorporation of the network modifier oxide which produces a weaker network strength and a larger polarizability.